Reproductive Physiology — MCQs

Reproductive Physiology Indian Medical PG Practice Questions and MCQs

Question 191: Which of the following reproductive cycles is absent in humans?

A. Menstrual cycle
B. Ovarian cycle
C. Hormonal cycle
D. Estrous cycle (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Estrous cycle**. **1. Why Estrous Cycle is the Correct Answer:** The estrous cycle is characteristic of most non-primate mammals (e.g., dogs, cows, rodents). Unlike humans, females in an estrous cycle are sexually receptive only during a specific phase called "estrus" (heat). Crucially, in the estrous cycle, the endometrium is **reabsorbed** by the uterus if fertilization does not occur, rather than being shed externally. Humans and higher primates exhibit the **menstrual cycle**, where the endometrium is shed (menstruation) and sexual receptivity is not strictly confined to a specific phase. **2. Why Other Options are Incorrect:** * **A. Menstrual Cycle:** This is the hallmark of human female reproduction, involving the monthly shedding of the functional layer of the endometrium due to the withdrawal of progesterone. * **B. Ovarian Cycle:** This refers to the rhythmic changes in the ovary, including the follicular phase, ovulation, and the luteal phase. It occurs concurrently with the uterine (menstrual) cycle in humans. * **C. Hormonal Cycle:** Human reproduction is governed by the rhythmic fluctuation of hormones (GnRH, FSH, LH, Estrogen, and Progesterone). This "hormonal cycle" drives both the ovarian and menstrual cycles. **Clinical Pearls for NEET-PG:** * **Silent Ovulation:** The first ovulation after puberty or the first one postpartum is often "silent" because it isn't preceded by a priming dose of progesterone, though the hormonal cycle is active. * **Luteal Phase:** This phase is remarkably constant in humans, lasting **14 days**. Variability in cycle length is usually due to variations in the follicular phase. * **Spontaneous vs. Induced Ovulators:** Humans are **spontaneous ovulators**, whereas animals like rabbits are **induced ovulators** (ovulation is triggered by coitus).

Question 192: Testosterone production is mainly contributed by which of the following cells?

A. Leydig cells (Correct Answer)
B. Sertoli cells
C. Seminiferous tubules
D. Epididymis

Explanation: **Explanation:** The production of testosterone is the primary endocrine function of the testes, specifically localized to the **Leydig cells** (also known as interstitial cells). These cells are situated in the connective tissue between the seminiferous tubules. 1. **Why Leydig Cells are Correct:** Leydig cells possess the necessary steroidogenic enzymes (such as 17β-HSD) to convert cholesterol into testosterone. This process is stimulated by **Luteinizing Hormone (LH)** from the anterior pituitary. LH binds to G-protein coupled receptors on Leydig cells, increasing cAMP and activating the rate-limiting step (StAR protein) for cholesterol transport. 2. **Why Other Options are Incorrect:** * **Sertoli Cells:** These are "nurse cells" located within the seminiferous tubules. Their primary role is supporting spermatogenesis. Under the influence of **FSH**, they produce Androgen Binding Protein (ABP), Inhibin B, and Anti-Müllerian Hormone (AMH), but they do not synthesize testosterone. * **Seminiferous Tubules:** This is the anatomical site for **spermatogenesis** (sperm production), not the primary site of hormone synthesis. * **Epididymis:** This is a ductal system responsible for the storage, transport, and **maturation** (gaining motility) of spermatozoa. **High-Yield Clinical Pearls for NEET-PG:** * **LH acts on Leydig cells** (Mnemonic: **L**H = **L**eydig). * **FSH acts on Sertoli cells** (Mnemonic: **F**SH = **S**ertoli). * **Blood-Testis Barrier:** Formed by tight junctions between Sertoli cells to protect developing germ cells. * **Testosterone Feedback:** High levels of testosterone provide negative feedback primarily on LH secretion (at both the hypothalamus and pituitary levels).

Question 193: Capacitation of sperm in the female reproductive tract helps sperm to:

A. Increase motility
B. Increase survival
C. Remove the glycoprotein coat from the acrosomal region (Correct Answer)
D. Release acrosin

Explanation: **Explanation:** **Capacitation** is the final step of sperm maturation that occurs within the female reproductive tract (primarily in the isthmus of the fallopian tube). It is a biochemical process lasting 1–7 hours, essential for the sperm to acquire the ability to fertilize an oocyte. **Why Option C is Correct:** The primary mechanism of capacitation involves the **removal of the glycoprotein coat and seminal plasma proteins** from the plasma membrane overlying the acrosomal region of the sperm. This process also involves the efflux of cholesterol, which increases membrane fluidity and permeability to calcium ions. These changes are prerequisites for the subsequent acrosome reaction. **Analysis of Incorrect Options:** * **Option A:** While capacitation leads to "hyperactivated motility" (whiplash-like movement), it is a *result* of the process rather than the definition. The fundamental change is the membrane modification. * **Option B:** Capacitation actually decreases the lifespan of the sperm. Once capacitated, the sperm becomes more fragile and must fertilize the egg quickly or it will perish. * **Option D:** The release of acrosin occurs during the **Acrosome Reaction**, which happens *after* capacitation is complete and the sperm binds to the Zona Pellucida (ZP3 receptor). **High-Yield NEET-PG Pearls:** * **Site:** Capacitation occurs in the female reproductive tract (uterus and fallopian tubes), NOT in the male tract. * **Calcium Role:** Capacitation is dependent on an influx of $Ca^{2+}$ and an increase in intracellular cAMP. * **Sequence:** Spermiogenesis (differentiation) $\rightarrow$ Maturation (Epididymis) $\rightarrow$ Capacitation (Female tract) $\rightarrow$ Acrosome Reaction (on contact with Zona Pellucida). * **In-vitro Fertilization (IVF):** In IVF, capacitation is artificially induced by washing the sperm in a protein-rich medium.

Question 194: All of the following are present in the acrosomal head of spermatozoa except?

A. Alkaline phosphatase (Correct Answer)
B. Acid phosphatase
C. Acrosomase
D. Hyaluronidase

Explanation: The acrosome is a lysosome-like organelle covering the anterior two-thirds of the sperm nucleus. It contains a cocktail of **hydrolytic and proteolytic enzymes** (collectively called sperm lysins) necessary for penetrating the egg's protective layers during fertilization. ### Why Alkaline Phosphatase is the Correct Answer **Alkaline phosphatase** is primarily found in the plasma membrane of cells, bone, liver, and the placenta, but it is **not** a constituent of the acrosomal vesicle. The acrosomal environment is specialized for acidic hydrolases. ### Explanation of Incorrect Options * **Acid Phosphatase:** Like other lysosomes, the acrosome contains acid phosphatase. It serves as a marker for lysosomal activity and helps in the breakdown of organic phosphates during the acrosomal reaction. * **Acrosomase (Acrosin):** This is a serine protease bound to the inner acrosomal membrane. It is essential for digesting the **zona pellucida**, allowing the sperm to reach the oocyte plasma membrane. * **Hyaluronidase:** This enzyme is released to dissolve the hyaluronic acid polymer in the ground substance of the **cumulus oophorus** (the cluster of cells surrounding the egg), enabling the sperm to navigate toward the zona pellucida. ### High-Yield Clinical Pearls for NEET-PG * **The Acrosome Reaction:** Triggered by binding to the ZP3 receptor on the zona pellucida; it requires an influx of **Calcium (Ca²⁺)**. * **Capacitation:** A functional maturation process occurring in the female reproductive tract (primarily the isthmus of the fallopian tube) that involves the removal of cholesterol and glycoproteins from the sperm head, making the acrosome membrane fluid enough for the acrosome reaction. * **Key Enzymes to Remember:** Hyaluronidase (for cumulus oophorus), Acrosin (for zona pellucida), and Neuraminidase.

Question 195: Inhibin-A is produced by?

A. Sertoli cell
B. Leydig cell
C. Hilus cell
D. Placenta (Correct Answer)

Explanation: **Explanation:** Inhibins are glycoprotein hormones that selectively inhibit the secretion of Follicle-Stimulating Hormone (FSH). They exist in two primary forms: **Inhibin A** and **Inhibin B**. 1. **Why Placenta is Correct:** During pregnancy, the **Placenta** (specifically the syncytiotrophoblast) is the primary source of **Inhibin A**. Its levels rise significantly during the first trimester and are used clinically in the **Quadruple Marker Test** to screen for Down Syndrome (Trisomy 21), where Inhibin A levels are characteristically elevated. In non-pregnant females, Inhibin A is produced by the dominant follicle and the corpus luteum. 2. **Why Other Options are Incorrect:** * **Sertoli Cells:** These cells in the testes primarily produce **Inhibin B**. Inhibin B serves as a marker of spermatogenesis and provides negative feedback on FSH in males. * **Leydig Cells:** These cells are responsible for testosterone production under the influence of LH; they do not produce significant amounts of inhibin. * **Hilus Cells:** Located in the ovarian hilum (homologous to Leydig cells), they secrete androgens and are not a source of inhibin. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibin A:** Think **A**fter ovulation (Corpus Luteum) and **A**bnormal pregnancy screening (Down Syndrome). * **Inhibin B:** Think **B**efore ovulation (Granulosa cells of pre-antral follicles) and **B**oy/Male (Sertoli cells). * **Tumor Marker:** Inhibins are used as specific tumor markers for **Granulosa Cell Tumors** of the ovary. * **Down Syndrome Screening:** The Quadruple screen includes: AFP (low), uE3 (low), hCG (high), and **Inhibin A (high)**.

Question 196: What is the uteroplacental blood flow per minute?

A. 200 ml/minute
B. 400 ml/minute
C. 300 ml/minute
D. 600 ml/minute (Correct Answer)

Explanation: **Explanation:** The correct answer is **600 ml/minute**. In a term pregnancy, the uteroplacental blood flow (UPBF) increases significantly to meet the high metabolic demands of the growing fetus and placenta. At term, the total uterine blood flow is approximately **500–750 ml/min**, which represents about 10–15% of the total maternal cardiac output. Of this, roughly 80–90% (approx. 600 ml/min) specifically supplies the intervillous space (placenta), while the remainder supplies the myometrium. **Analysis of Options:** * **Option A (200 ml/min):** This value is too low for a term pregnancy; it is more representative of uterine blood flow in the mid-second trimester. * **Option B & C (300–400 ml/min):** While blood flow increases progressively throughout pregnancy, these values represent the flow around 28–32 weeks of gestation, not the peak flow at term. * **Option D (600 ml/min):** This is the standard physiological value cited in major textbooks (like Ganong and Guyton) for uteroplacental flow at term. **High-Yield Clinical Pearls for NEET-PG:** * **Regulation:** Uterine blood flow is **not autoregulated**; it depends directly on maternal mean arterial pressure and is inversely proportional to uterine vascular resistance. * **Vascular Remodeling:** The physiological conversion of high-resistance spiral arteries into low-resistance, high-capacity vessels (mediated by trophoblast invasion) is essential for achieving this high flow rate. * **Clinical Correlation:** Failure of this remodeling leads to reduced UPBF, resulting in conditions like **Preeclampsia** and **IUGR** (Intrauterine Growth Restriction). * **Positioning:** Maternal supine position can decrease UPBF due to aortocaval compression (Supine Hypotension Syndrome).

Question 197: Successful fertilization is most likely to occur when the oocyte is in which stage of development and location?

A. The oviduct and has entered the second meiotic division (Correct Answer)
B. The uterus and has completed the first meiotic division
C. Metaphase of mitosis
D. The graafian follicle, which then enters the oviduct

Explanation: **Explanation:** The correct answer is **A: The oviduct and has entered the second meiotic division.** **1. Why Option A is Correct:** In humans, ovulation releases a **secondary oocyte** that has completed Meiosis I and is arrested in **Metaphase of Meiosis II**. Fertilization typically occurs in the **ampulla of the fallopian tube (oviduct)**. The oocyte only completes the second meiotic division *after* a sperm successfully penetrates its zona pellucida, triggering the release of the second polar body. Therefore, at the moment of successful fertilization, the oocyte is physically in the oviduct and chronologically in the second meiotic division. **2. Why Other Options are Incorrect:** * **Option B:** The uterus is the site of implantation (blastocyst stage), not fertilization. If an unfertilized egg reaches the uterus, it is usually degenerating. * **Option C:** Oocytes undergo **meiosis**, not mitosis. Mitosis is characteristic of somatic cells and the early cleavage stages of the zygote. * **Option D:** While the oocyte develops in the Graafian follicle, fertilization cannot occur there. The follicle must rupture (ovulation) to release the oocyte into the oviduct where it meets the capacitated sperm. **3. NEET-PG High-Yield Pearls:** * **Meiotic Arrests:** The 1st arrest occurs in **Prophase I (Diplotene stage)** at birth. The 2nd arrest occurs in **Metaphase II** at ovulation. * **Fertilization Site:** The **Ampulla** is the most common site for both fertilization and ectopic pregnancies. * **Sperm Capacitation:** This 7-hour process occurs in the female reproductive tract (mainly the uterus and oviduct) and is essential for the sperm to acquire fertilizing capacity. * **Cortical Reaction:** Triggered by sperm entry to prevent **polyspermy** by hardening the zona pellucida.

Question 198: The male sex hormone testosterone is produced by which cells?

A. Sertoli cells
B. Epithelial cells
C. Interstitial cells of Leydig (Correct Answer)
D. Primitive germ cells

Explanation: **Explanation:** The production of testosterone is the primary endocrine function of the testes, carried out by the **Interstitial cells of Leydig**. These cells are located in the connective tissue spaces between the seminiferous tubules. **1. Why Option C is Correct:** Leydig cells possess specific receptors for **Luteinizing Hormone (LH)** secreted by the anterior pituitary. When LH binds to these receptors, it activates the cAMP pathway, leading to the conversion of cholesterol into pregnenolone and ultimately **testosterone**. This process is essential for the development of male secondary sexual characteristics and the stimulation of spermatogenesis. **2. Why Other Options are Incorrect:** * **Sertoli Cells:** These are "nurse cells" located within the seminiferous tubules. Their primary role is to support and nourish developing sperm. They secrete **Inhibin** (which inhibits FSH) and **Androgen Binding Protein (ABP)**, but they do not synthesize testosterone. * **Epithelial Cells:** In the context of the testes, the germinal epithelium consists of Sertoli cells and spermatogenic cells. While they form the structure of the tubules, they lack the enzymatic machinery to produce steroid hormones. * **Primitive Germ Cells (Spermatogonia):** These are the undifferentiated stem cells that undergo meiosis to eventually become mature spermatozoa. They are the *target* of hormonal action rather than the source. **High-Yield Clinical Pearls for NEET-PG:** * **LH acts on Leydig cells** (Mnemonic: **L**H = **L**eydig). * **FSH acts on Sertoli cells** (Mnemonic: **F**SH = **S**ertoli). * **Blood-Testis Barrier:** Formed by tight junctions between Sertoli cells. * **Testosterone Feedback:** Testosterone provides negative feedback primarily on **LH** secretion at the pituitary and GnRH at the hypothalamus.

Question 199: Human chorionic gonadotropin shares structural similarity with all except?

A. FSH
B. GH (Correct Answer)
C. LH
D. TSH

Explanation: ### Explanation The correct answer is **GH (Growth Hormone)**. **1. Underlying Medical Concept:** Human Chorionic Gonadotropin (hCG) belongs to the **glycoprotein hormone family**, which also includes Luteinizing Hormone (LH), Follicle-Stimulating Hormone (FSH), and Thyroid-Stimulating Hormone (TSH). All these hormones are heterodimers consisting of two subunits: * **Alpha ($\alpha$) subunit:** Identical in all four hormones (encoded by the same gene). * **Beta ($\beta$) subunit:** Unique to each hormone, providing biological and receptor specificity. **hCG and LH** share the highest degree of structural similarity (approximately 80% of their amino acid sequence in the $\beta$-subunit is identical). This explains why hCG can bind to the LH receptor to maintain the corpus luteum during early pregnancy. **2. Why the other options are incorrect:** * **FSH and TSH:** As members of the glycoprotein family, they share the exact same $\alpha$-subunit as hCG. Therefore, they possess significant structural similarity. * **GH (Growth Hormone):** GH is a **single-chain polypeptide** hormone (not a glycoprotein) and belongs to the Somatotropin family (along with Prolactin and Human Placental Lactogen). It does not share the $\alpha/\beta$ dimer structure, making it structurally distinct from hCG. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cross-reactivity:** Because hCG and TSH share the $\alpha$-subunit, very high levels of hCG (as seen in Hydatidiform mole or Choriocarcinoma) can weakly stimulate TSH receptors, leading to **gestational hyperthyroidism**. * **Biological Half-life:** hCG has a much longer half-life (~24 hours) compared to LH (~20 minutes) due to a highly glycosylated C-terminal tail on its $\beta$-subunit. * **Pregnancy Tests:** Immunological pregnancy tests specifically detect the **$\beta$-subunit of hCG** to avoid cross-reactivity with LH, FSH, or TSH.

Question 200: Fertilization occurs in which part of the fallopian tube?

A. Fimbrial
B. Isthmus
C. Ampulla (Correct Answer)
D. Interstitial

Explanation: **Explanation:** **1. Why Ampulla is Correct:** The **Ampulla** is the widest and longest part of the fallopian tube, making it the most common site for fertilization. It possesses a highly folded mucosal lining (plicae) and a rich supply of ciliated cells. These features create an ideal environment for the sperm and ovum to meet. Fertilization typically occurs here within 12–24 hours after ovulation. **2. Analysis of Incorrect Options:** * **Fimbriae (A):** These are finger-like projections at the distal end of the tube that "sweep" the ovum from the ovary into the infundibulum. They facilitate capture but are not the site of fertilization. * **Isthmus (B):** This is the narrow, thick-walled segment medial to the ampulla. While sperm undergo final maturation (capacitation) here, it is generally too narrow for the primary fertilization event. * **Interstitial/Intramural (D):** This is the segment that traverses the uterine wall. It is the narrowest part of the tube (0.7 mm) and is a rare but dangerous site for ectopic pregnancies. **3. NEET-PG High-Yield Pearls:** * **Ectopic Pregnancy:** The **Ampulla** is also the most common site for ectopic pregnancy (approx. 70-80%). * **Ciliary Action:** The movement of the ovum toward the uterus is mediated by ciliary beat and peristalsis, whereas sperm move primarily via their own motility and uterine contractions. * **Time Window:** The secondary oocyte remains viable for 12–24 hours, while sperm can survive in the female reproductive tract for 48–72 hours. * **Narrowest Part:** The **Interstitial** portion is the narrowest part of the fallopian tube, while the **Isthmus** is the narrowest part of the extra-uterine tube.

Practice by Chapter

Male Reproductive Physiology

Practice Questions

Spermatogenesis and Sperm Function

Practice Questions

Female Reproductive Physiology

Practice Questions

Menstrual Cycle

Practice Questions

Ovulation and Fertilization

Practice Questions

Physiology of Pregnancy

Practice Questions

Parturition

Practice Questions

Lactation

Practice Questions

Sexual Differentiation and Development

Practice Questions

Reproductive Aging

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